1. Field of the Invention
The present invention relates to a conveying apparatus and, in particular, to a conveying apparatus suitable for conveying magnetic workpieces such as lead frames.
2. Description of the Related Art
As a conventional example of a lead frame conveying apparatus, a guiding apparatus has been proposed in Japanese Utility Model Publication No. 7-35388, in which a lead frame is guided along the upper surface of a non-magnetic guide rail and in which a non-magnetic guide member is provided at one side edge of the upper surface of the guide rail. The apparatus also has magnets embedded in the guide member at appropriate intervals. In this apparatus, the side edge of the lead frame is attracted by the magnetic force and kept in contact with the guide member, so that a high level of positional accuracy can be achieved. Further, there is no need to provide a device for preventing the lead frame from going backward or a device for imparting tension.
In the above-described apparatus, however, it is necessary to employ transfer means such as a transfer lug for transferring the lead frame, and it is difficult to perform high-speed conveying with low vibration and low noise. Further, since the lead frame is in close contact with the guide member, there is frictional resistance between the lead frame and the guide member. When the transfer lug is engaged with the lead frame to transfer the lead frame, a large load is partially applied to the lead frame, and there is a danger of the lead frame being expanded or deformed.
To solve the above problem, U.S. Pat. No. 5,816,385 proposes a conveying apparatus capable of conveying a magnetic workpiece at high speed with low vibration and low noise and allowing high-accuracy positioning to be effected. This conveying apparatus serves to convey a magnetic workpiece having two adjacent surfaces. The apparatus comprises a non-magnetic rail having a guide surface for slidably guiding a first surface of the workpiece. The conveying apparatus also comprises a non-magnetic belt having a conveying surface in contact with a second surface of the workpiece and movable along the rail and driving means for running the belt. The conveying apparatus further comprises a magnet which is arranged at a position opposed to the rail through the intermediation of the belt and which generates a magnetic force including a component force for bringing the second surface of the workpiece into close contact with the belt and a component force for bringing the first surface of the workpiece into contact with the rail.
In the above conveying apparatus, the magnetic workpiece is conveyed in a state in which it is attracted to a magnet (yoke) through the intermediation of the non-magnetic belt. When conveying a workpiece requiring a large magnetic force, the friction between the non-magnetic belt and the magnet (yoke) increases. This friction leads to wear of the non-magnetic belt and the magnet (yoke) which results in a deterioration in durability and a degeneration in conveying position accuracy of the apparatus. Furthermore, a large driving force is required to drive the belt.
The wear on the non-magnetic belt and the magnet (yoke) may be avoided to some extent by performing low-friction surface treatment on the sliding surface of the non-magnetic belt or the yoke, or applying oil for lubrication. These solutions, however, are not permanent measures to solve the problem.